The invention refers to a multitubular bag for electrodes of electric batteries, made of synthetic fabric and foreseen for holding active material of positive plates of electric lead-acid batteries.
It is known that the duration of electric lead batteries is directly related to the removal of active material from the positive plate and to the corrosion of the conducting support, made of lead alloy, of the same positive plate. Both of these effects are caused by mechanical stresses that are related to volume changes of the active material, with the surcharging of elements and vibrations to which the battery is sometimes subjected.
For avoiding rapid depletion of electric batteries the indicated drawbacks have been limited by developing tubular positive plates, in which the conductive support made of lead alloy, a pin, is housed in the center of the active material with a limited surface exposed to anodic corrosion and with the active material kept pressed and compacted around the pin by means of pipes made of porous material which is resistant to acids.
These pipes are made in the form of multitubular bags, which at the end of the machinings have dimensions and pipe number related to the characteristics of the tubular positive plates to be obtained.
Further it is known that the plate capacity is related to the quantity of active material and the possibility for this active material being contacted from the acid at the required surface, while any phenomenon that can slow down or reduce the diffusion of the acid causes, as a result, a higher voltage drop, therefore a negative effect on the high efficiency of the same electric battery. These voltage drops are more detrimental in the case of drive type electric batteries where the discharges have a high intensity (that is a discharge occurs each 24 hours). The stationary electric batteries have mainly a slow discharge, although in certain applications there are also foreseen short discharges with high intensity in electric batteries of this type.
Therefore the above mentioned pipes that keep the active material pressed and compacted around the pinsmust have precise mechanical and chemical characteristics. From a mechanical point of view the pipes must have the following characteristcs:
a very high porosity and preferably similar to a microporous structure for holding better the active material, in particular when this is made of extremely thin powder;
a good elasticity, keeping in mind that the activematerial in the inside of pipes suffers volume variations under the effect of chemical transformations to which it is submitted;
a good resistance to abrasion, this last being a consequence of vibrations to which the drive type electric batteries are subjected during the service;
a good mechanical resistance constant versus time for guaranteeing the compactness of the positive active material and its coherence with the conductive lead core inserted in the center of the same pipe.
From the chemical point of view the required characteristics are the following ones:
a high resistance to oxidation, that for drive electric batteries must be particularly high, these electric batteries being submitted to continuous cycles of charging and discharging with severe runnings during which nascent oxygen is generated on the positive plate;
minimum pollution of the electrolyte caused by products of breakdown of polymeric molecules, in particular with respect to chlorine or perchlorate ions.
Different embodiments of these pipes and related multitublar bags are known, e.g. there are known pipes made of real fabric, in particular of fabric based on polyester fibers, or pipes made of "non-woven" fabric, that is of felt, usually still made of polyester.
The pipes made of real fabric based on polyester fibers have a good porosity and excellent mechanical characteristics, good elasticity, do not release polluting materials and have a sufficient resistance to oxidation. They are widely used in the sector of multitubular bags made of polyester fibers in continuous high tenacity filament with excellent mechanical characteristics.
Yet the porosity is supplied by openings that are obtained in the weaving process with the free spaces between yarns, and the high tenacity polyester, being usually coated with resins, has a very smooth surface which can cause difficulties in holding very thin powders.
The pipes of "non-woven" fabric made of felt, usually made of polyester, have naturally an excellent porosity, but have limitations owing to the poor mechanical strength of felt and lack of elasticity.
Further owing to the number of charging and discharging cycles of the electric battery, it is noticed a continuous relaxation of the same pipes and therefore a consequent loss of compression and coherence of the active material and above all of the coherence of the same active material with the conductive pin made of lead alloy.